Responses of generalist invertebrate predators to pupal densities of autumnal and winter moths under field conditions

Abstract

Abstract. Generalist natural enemies are usually not considered as being capable of causing population cycles in forest insects, but they may influence the population dynamics of their prey in the low density cycle phase when specialist enemies are largely absent. In the present field study, the total response of the generalist invertebrate predator community to experimentally established pupal densities of the closely related autumnal (Epirrita autumnata) and winter moths (Operophtera brumata) was analysed. Due to the high amount of variation in the dataset, the exact shape of the response curve could not be convincingly estimated. Nevertheless, two important conclusions can be drawn from the analyses. Firstly, the natural invertebrate predator community seems to become saturated at rather low densities of both autumnal and winter moth pupae. Secondly, the predator community seems to become saturated at much lower densities of autumnal than of winter moth pupae. Furthermore, pupal mass was significantly negatively correlated with invertebrate predation probability in autumnal moth pupae. These results indicate that differences in the predator assemblage being able to consume pupae of the two moth species, as well as different handling times, could be responsible for the substantially higher predation rates in winter than in autumnal moth pupae. As a consequence, the population dynamics of autumnal moths might be less affected by generalist invertebrate predators than those of winter moths, as autumnal moths seem able to escape from the regulating influence of generalist predators at much lower population densities than winter moths.